In Fourier transform infrared spectroscopy, an interferometer which includes a beam splitter, fixed mirror and movable mirror is used, such as a Michelson's interferometer. In an interferometer, the position of the movable mirror is changed to change the optical length difference between a beam of light reflected by this movable mirror and a beam of light reflected by the fixed mirror, and thereby generate varying interfering light in which the two aforementioned beams of light interfere with each other having a phase difference which varies depending on the optical length difference. Infrared light having a wavelength width including an absorption wavelength of a target compound contained in a sample is introduced into the interferometer to generate interfering light. This light is cast into the sample, and the intensity of the transmitted light is measured. Such a sequence of operations is repeatedly performed while gradually changing the position of the movable mirror. As a result, an interferogram which shows the change in the intensity of the transmitted light with respect to the moving distance of the movable mirror is obtained (FIG. 1). By Fourier-transforming this interferogram, a power spectrum with the vertical axis indicating the intensity and the horizontal axis indicating the wavenumber can be obtained (for example, see Patent Literature 1, 2 or 3).
FIG. 2 shows the configuration of the main components of a conventionally used Fourier transform infrared spectrophotometer. The Fourier transform infrared spectrophotometer 100 is roughly composed of two sections: an interference section located within an airtight chamber 101 and a measurement section located outside the airtight chamber 101. The interference section includes a light source 102, condensing mirror 103, collimating mirror 104, beam splitter 105, fixed mirror 106, and movable mirror 107. The measurement section includes a parabolic mirror 112, sample chamber 113, ellipsoidal mirror 115, and detector 116. A sample 114 is placed within the sample chamber 113.
Infrared light emitted from the light source 102 is cast onto the beam splitter 105 via the condensing mirror 103 and the collimating mirror 104. The beam splitter 105 splits the light into two beams travelling toward the fixed and movable mirrors 106 and 107, respectively. The infrared beams respectively reflected by the fixed and movable mirrors 106 and 107 return to the bean splitter 105 and are merged into a single beam. This beam exits from the window 108 of the airtight chamber 101 and travels toward the parabolic mirror 112. Being condensed by the parabolic mirror 112, the beam is cast into the sample 114 inside the sample chamber 113. After passing through the sample 114, the beam is focused onto the detection surface of the detector 116 by the ellipsoidal mirror 115 and detected. Driving the movable mirror 107 back and forth (in the direction indicated by arrow M in FIG. 2) causes a change in the optical length difference between the infrared light reflected by the fixed mirror 106 and the infrared light reflected by the movable mirror 107. As a result, the two beams of infrared light interfere with each other having a phase difference which changes depending on the optical length difference. The interfering infrared light transmitted through the sample 114 is detected by the detector 116.
An example of the detector 116 is a pyroelectric detector having a pyroelectric element and junction field-effect transistor. The pyroelectric element generates electric charges in an amount corresponding to the amount of incident infrared light and generates an electric current. The pyroelectric element is connected to the gate of the junction field-effect transistor, while a resistor is connected to the source of the same transistor. Upon incidence of infrared light, a change in voltage occurs at the gate to which the pyroelectric element is connected, causing a change in the amount of electric current flowing out of the source. This in turn changes the potential difference between the two ends of the resistor connected to the source. By measuring this potential difference, a voltage value which corresponds to the amount of incident infrared light can be obtained. When there is no incident infrared light, a voltage which corresponds to the leakage current from the drain to the gate and the resistance of the pyroelectric element is applied to the gate. This means that an offset voltage is constantly generated between the two ends of the resistor. The pyroelectric detector produces an output voltage having a positive or negative value which represents a change from this offset voltage (see the waveform indicated by the solid line in FIG. 3). For distinction from the interferogram shown in FIG. 1, the waveform shown in FIG. 3 is hereinafter called the “subtracted waveform”.